1. Field of the Invention
This invention generally relates to stringed instruments such as guitars, and in particular, to piezoelectric transducers that are incorporated into bridges to produce electric signals upon detection of vibrations of strings.
This application claims priority on Japanese Patent Application No. 2002-346195, the content of which is incorporated herein by reference.
2. Description of the Related Art
Recently, in the fields of electronic musical instruments, so-called ‘sound mute type’ musical instruments are becoming widespread in the market because they allow users (or players) to play or practice musical instruments in restricted environments that do not allow generation of musical tones at high tone volumes.
FIG. 9 shows an example of the constitution of a guitar having sound mute function (simply referred to as a sound mute guitar). That is, a sound mute guitar 100 is constituted by nylon strings 110, a bridge 120 for supporting the nylon strings 110, a center body 130 for supporting the bridge 120, and a pair of frames 140 that form parts of the whole body and are arranged to the left and the right of the center body 130. The sound mute guitar 100 shown in FIG. 9 does not comprise a resonating body, which is normally provided for a general-use acoustic guitar. For this reason, even when the user (or player) plucks the nylon strings 110, the sound mute guitar 100 produces musical tones at very small tone volumes compared with normal tone volumes produced by playing a general-use acoustic guitar. In addition, the sound mute guitar 100 comprises a piezoelectric transducer that is arranged inside of the bridge 120 so as to produce electric signals upon detection of vibrations of the nylon strings 110. Furthermore, the sound mute guitar 100 comprises electric circuitry that is arranged inside of the center body 130 so as to amplify electric signals output from the piezoelectric transducer, so that amplified electric signals are supplied to a headphone terminal (or a headphone jack, not shown). Thus, even though the sound mute guitar 100 actually produces musical tones at very small tone volumes in the surroundings, the user of the sound mute guitar 100 can use a headphone set 150 to listen to musical tones with live-audio presence that the user may experience actually playing an acoustic guitar.
FIG. 10 is a cross sectional view taken along line A—A in FIG. 9 in proximity to the bridge 120 of the sound mute guitar 100. FIG. 11 is an exploded perspective view showing parts of the bridge 120 that are isolated from each other in illustration.
As shown in FIGS. 10 and 11, the bridge 120 is constituted by a support member 111 for supporting the strings 110, a bridge base 112 having an elongated hollow 112a for fixing the support member 111 therein, and a piezoelectric transducer 113 that is arranged between the support member 111 and the bridge base 112.
The piezoelectric transducer 113 is formed in a thin elongated rectangular shape that substantially matches the bottom shape of the elongated hollow 112a of the bridge base 112. Reasons why the piezoelectric transducer 113 is roughly formed in an elongated rectangular shape is to uniformly detect vibrations of all the strings 110.
FIG. 12 shows a cross-sectional constitution of the piezoelectric transducer 113, which is constituted by various layers arranged between an upper surface (facing the support member 111) and a lower surface (facing the bottom of the elongated hollow 112a of the bridge base 112). That is, the piezoelectric transducer 113 is constituted by a polymeric piezoelectric film 113a composed of polyvinylidene fluoride (PVDF), a pair of electrodes 113b respectively adhered to the upper surface and the lower surface of the polymeric piezoelectric film 113a, and a pair of insulation sheets 113c composed of vinyl chloride and the like, all of which are completely covered with a conductive shield (or a shield layer) 113d. The insulation sheets 113c are arranged to insulate the electrodes 113b from the conductive shield 113d. Herein, the conductive shield 113d is composed of a prescribed material such as aluminum and copper, details of which are disclosed in Japanese Patent Application Publication No. Hei 7-160265 (in particular, page 3 and FIG. 2), for example.
As described above, it is necessary to cover the polymeric piezoelectric film 113a and its related layers with the conductive shield 113d because the polymeric piezoelectric film 113a composed of PVDF and the like normally has a high impedance and may easily pick up external noise such as hum noise (or humming noise). That is, in order to reduce the influence of external noise, the polymeric piezoelectric film 113a is covered with the conductive shield 113d. 
The most important point that the user (or player) may regard as an important factor in playing the sound mute guitar 100 is the tone color of the sound mute guitar 100 that can be heard via the headphone set 150 and the like. It is a present problem that the tone color of the sound mute guitar 100 may be greatly influenced by the piezoelectric transducer 113. The manufacturer which produces the sound mute guitar 100 may actualize a high-quality tone color by installing a high-performance piezoelectric transducer having a desired characteristic in the sound mute guitar 100. However, such a piezoelectric transducer is very expensive and reduces productivity. For this reason, the manufacturer of the sound mute guitar 100 must use a relatively inexpensive piezoelectric transducer, which can be produced at a relatively high productivity, by sacrificing important factors such as tone color of the sound mute guitar 100.